Some of them are selling (or claim to be selling) American public domain books from Project Guttenberg if that's what you mean, but the same comment would apply -- they have no right to sell them outside America, and anyone who wants them can just download them for free instead.

Completely incorrect.

If a book is PD in your country, then you are entitled to do whatever you like, including selling it. Yes, people can download it for free, but you can sell it if you wish. There is nothing legally wrong with this, unless you're actively advertising it in countries where it's not PD.